Mobile construction machines, such as excavators, include multiple hydraulic circuits. For example, each machine can have a travel circuit and at least one implement circuit. The travel circuit typically includes an engine-driven pump connected to provide pressurized fluid to a traction motor in a closed configuration. The implement circuit typically includes another engine-driven pump connected to one or more implement actuators (e.g., motors and/or cylinders) in an open configuration. Each of the pumps of the different circuits is generally sized to consume near full engine power, such that the associated actuator(s) can operate at maximum speed and/or with maximum force when commanded to do so by the operator. In most instances, however, the various circuits are not commanded to operate at maximum capacity. En fact, the circuits of a particular machine are most often commanded to operate at only half capacity or less. Accordingly, the conventional circuits may be overdesigned, resulting in a high-cost, bulky, and inefficient system. In addition, care must be taken during combined circuit use, such that the power drawn by the various circuits does not exceed a capacity of the engine to provide power and thereby cause the engine to stall.
One attempt to address the issues discussed above is disclosed in U.S. Patent Publication 2014/0020370 of Berg that published on Jan. 23, 2014 (the '370 publication). In the '370 publication, a hydrostatic drive system is disclosed for use in a vehicle. The system includes an open circuit with a main hydraulic pump that is engine-driven. The main hydraulic pump is configured to drive one or more implement cylinders. The system also includes a closed circuit with an auxiliary hydraulic pump that is also engine-driven. The auxiliary hydraulic pump is configured to drive a travel motor of the vehicle. The system further includes a combiner valve configured to divert fluid from the open circuit to the closed circuit, and a relief valve configured to drain fluid from the dosed circuit that exceeds a maximum capacity of the auxiliary pump. By selectively sharing fluid between the open and closed circuits, the vehicle may be able to travel at higher speeds while using a smaller auxiliary pump. This may result in a smaller system, lower cost, and greater efficiency.
While the system of the '370 publication provides improvement over the prior art, the system may still be less than optimal. In particular, because the system utilizes an open circuit, an overall efficiency of the system may be lower than desired for some applications. In addition, the relief valve used to drain fluid from the closed circuit may be oversized in order to handle the normal flow of the closed circuit and the periodic combined flows from the open and closed circuits. This oversizing of the relief valve may further decrease an efficiency of the system and increase a cost of the system.
The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth above and/or other problems of the prior art.